Counties in Colorado by share of days with good air quality (US EPA AQI)

Alamosa County had the highest share of days with good air quality (US EPA AQI) of 31 counties in 2019, at 100%. Denver County had the lowest, at 38.1%. The median was 72.9%. Source: EPA.

Highest100Alamosa County

Lowest38.1Denver County

Median72.931 counties

Map of counties, 2019

Under 53
53 to 65
65 to 73
73 to 82
82 to 99
99 and over
No data

%, 2019. Hatched areas have no value for this period. Source: Invokely calculation with data from EPA. Boundaries: U.S. Census Bureau cartographic boundary files.

All counties, 2019

%. Source: Invokely calculation with data from EPA. Retrieved 2026-10-12.

Full ranking

Rank	Counties	2019 (%)	Change on 2018
1	Alamosa County	100	+3.3 pts
1	Fremont County	100	0 pts
1	Jackson County	100	+2.6 pts
1	Pitkin County	100	+2.4 pts
1	San Juan County	100	+7.6 pts
6	Pueblo County	99.2	+11.4 pts
6	Routt County	99.2	-0.5 pts
8	Prowers County	99.1	+0.8 pts
9	Delta County	92.6	+3.7 pts
10	Archuleta County	91.2	0 pts
11	Gilpin County	81.8	n/a
12	Grand County	80.5	+14.2 pts
13	San Miguel County	78.4	+11.2 pts
14	Montezuma County	76.4	+4.3 pts
15	La Plata County	73.2	+8.5 pts
16	Gunnison County	72.9	+8.5 pts
17	Rio Blanco County	69	+10.1 pts
18	El Paso County	68.2	+8.7 pts
19	Garfield County	67.9	+9.3 pts
20	Mesa County	65.4	+12.5 pts
21	Jefferson County	65.2	+2.5 pts
22	Clear Creek County	62.2	+12.4 pts
23	Douglas County	60.9	+7.6 pts
24	Arapahoe County	60.8	-1.4 pts
25	Chaffee County	55.6	+14.7 pts
26	Boulder County	52.6	-2.1 pts
27	Park County	51.3	+1.6 pts
28	Adams County	50.1	+12.3 pts
28	Larimer County	50.1	+6.3 pts
30	Weld County	41.6	+3.8 pts
31	Denver County	38.1	+4.1 pts

Other periods

About this ranking

Days with a daily Air Quality Index of 0 to 50 (good) as a percentage of the days of the year with an AQI in a US county. Calculated by Invokely from the day counts in the EPA AirData annual summaries; both counts are stored with each value.

United States counties with air quality monitors only, about 1,000 of some 3,100 in recent years; a county without monitors has no value. The number of days with an AQI differs between counties because some monitors run only in the ozone season.

Every value in the ranking comes from the same publisher and the same period, so the counties are compared on one definition. 9 counties with data in other periods have no value for 2019 and are left out rather than filled from another source. Equal values share a rank.

How to cite

U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: good days as a share of days with an AQI, calculated with stored counts. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Retrieved 2026-10-12.

Citation styles

The publisher is the author. Invokely is named only as the place the figures were read, where a style asks for it.

APA 7

U.S. Environmental Protection Agency. (2026). EPA AirData annual AQI by county: good days as a share of days with an AQI, calculated with stored counts (Counties in Colorado by share of days with good air quality (US EPA AQI), 2019) [Data set]. Retrieved October 12, 2026, from https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual

MLA 9

EPA AirData annual AQI by county: good days as a share of days with an AQI, calculated with stored counts, Counties in Colorado by share of days with good air quality (US EPA AQI), 2019. U.S. Environmental Protection Agency, 13 July 2026, https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Invokely, https://invokely.ai/rankings/good-air-quality-days-share/colorado-counties?period=2019. Accessed 12 Oct. 2026.

Chicago (author-date)

U.S. Environmental Protection Agency. 2026. EPA AirData annual AQI by county: good days as a share of days with an AQI, calculated with stored counts. Counties in Colorado by share of days with good air quality (US EPA AQI), 2019. Dataset. Accessed October 12, 2026. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Also distributed by Invokely, https://invokely.ai/rankings/good-air-quality-days-share/colorado-counties?period=2019.

Harvard

U.S. Environmental Protection Agency (2026) EPA AirData annual AQI by county: good days as a share of days with an AQI, calculated with stored counts, Counties in Colorado by share of days with good air quality (US EPA AQI), 2019 [Dataset]. Available at: https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual, via Invokely: https://invokely.ai/rankings/good-air-quality-days-share/colorado-counties?period=2019 (Accessed: 12 October 2026).

BibTeX

@misc{environmental_counties_in_colorado_by_2019,
  author = {{U.S. Environmental Protection Agency}},
  title = {{EPA AirData annual AQI by county: good days as a share of days with an AQI, calculated with stored counts: Counties in Colorado by share of days with good air quality (US EPA AQI), 2019}},
  year = {2026},
  howpublished = {Data set},
  url = {https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual},
  urldate = {2026-10-12},
  note = {Accessed via Invokely, https://invokely.ai/rankings/good-air-quality-days-share/colorado-counties?period=2019. Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: good days as a share of days with an AQI, calculated with stored counts. https://aqs.epa.gov/aqsweb/airdata/download\_files.html\#Annual. Retrieved 2026-10-12.},
}

RIS

TY  - DATA
AU  - U.S. Environmental Protection Agency
TI  - EPA AirData annual AQI by county: good days as a share of days with an AQI, calculated with stored counts: Counties in Colorado by share of days with good air quality (US EPA AQI), 2019
PY  - 2026
DA  - 2026/07/13
PB  - U.S. Environmental Protection Agency
DB  - Invokely
UR  - https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual
L2  - https://invokely.ai/rankings/good-air-quality-days-share/colorado-counties?period=2019
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/good-air-quality-days-share/colorado-counties?period=2019.
N1  - Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: good days as a share of days with an AQI, calculated with stored counts. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Retrieved 2026-10-12.
ER  -

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Questions

Which county has the highest share of days with good air quality (US EPA AQI)?

Alamosa County, with 100% in 2019, according to EPA.

Which county has the lowest share of days with good air quality (US EPA AQI)?

Denver County, with 38.1% in 2019.

Where does this ranking come from?

Every value comes from one source and one period, so the places are comparable: U.S. Environmental Protection Agency, EPA AirData annual AQI by county: good days as a share of days with an AQI, calculated with stored counts. Retrieved 2026-10-12. 9 counties have no value for 2019 and are not ranked.